notes on the natural history of aldabra, assumption

NOTES ON THE NATURAL HISTORY OF ALDABRA, ASSUMPTION AND GLORIOSO ISLANDS, INDIAN OCEAN.

BY

Dr.

W.

L. Abbott.*

The

atoll of Aldabra lies220 miles northwest of the north point of Madagascar,inlatitude'.1°25'south,

and

longitude 1<Peast. Itis about 22 miles long-

by

8 milesin extremewidth, the long axis lying east

and

west.t It is entirelyof*coral formation,

and

forms anovalring ofland, broken atseveralpoints

by

channels,and inclosing;i lagoon.

The

ring ofdry landiswidestatthe southeast

and

northwestcorners,whereit is

nearly 3 miles across.

The Grande

Terre, or

main

island,forms three- fifths of the circumference of the ring. It includes (from

midway

on the western side of the ring) the whole southern

and

eastern sides to apoint on the north shore, being 35 miles long. It is separated

by

Pass Hourreau, 200yardswide,from

North

orMiddleIsland. This

is 12 mileslong, formingthe north shore as faras

Grand

Pass. Thisis

the principalopeningintothe lagoon. It is400yards wide,

and

8 to 10 fathoms deep.

West

of this lies lie Picard, or Northwest Island, formingthe northwesterncorner of the atoll. Itis about5 mileslong.

Between

the south endof lie Picard

and

the northwestend of

Grande

Terre, lie half a dozen small islands

and

as

many

shallow channels.

The

lagoon is about20miles long

and

(5 milesin width. Exceptingin the northwestern corner near

Grand

Pass

and

in a- fewchannels,it is

veryshallow, half of it being nearly dryat low ^tide.

Grand

Passis

theonlyinletdeep

enough

to allow thepassageofa largevessel,

and

through this the currentrunswith great rapidity, 5to7knots, so that

itisdangerous for sailing vessels except atthe turn of the tides.

At

Pass

Hourreau

there is a narrow channel, through which a small vessel mightpass. .

The

innerorlagoonshore of the landiseverywhere

bounded by mangrove swamps,

intersected

by numerous

channels.

During

thenorthwest

monsoon

a

heavy

swell ^rolls in through

Grand

Pass

and

breaks

upon

thereef within the lagoon. It is verydanger- oustoboats at such times,

and

the pass

cannot

betraversed. There are

numerous

islandsscattered about the lagoon, the longest beinglie Sepoy, about5 milesfrom

Grand

Pass

and

directlyopposite toit, and

lieMichel, oppositeto Pass Hourreau, and close to the southern side

"Edited by Frederick \Y. True, with tbe assistance of other curators of the Museum. Noidentificationsofspecies were included in Dr.Abbott'smanuscript.

tTheisland wascompletely surveyed by II. M. S. Aleft, in 1882.

Proceedings National Museum, Vol. XVI—No.973.

759

760 NATURAL HISTORY OF ALDABRA ISLAND — ^ABBOTT.

of the lagoon. There are hundreds of other smaller islets, varying

in sizefrom a lew acresto a lew square yards.

The

atollisentirelyofcoralformation. Darwin,inhis "Coral Reefs,"

relyingprincipally on the reports of Capt. Moresby, did not regard it

asa trueatoll.

The

roeks of whiehitis

composed

weresaid tobe "vitri- fied."

The

rock certainly resembleslavainits

outward

appearance

;

but ^it is easily broken, and the fracture displays a white interiorand

numerous

fossil corals that areina scarcelyaltered ^slate.

The

rock gives a peculiar ringing sound

when

struck.

The

principal difference between Aldabra and other coral islands is.

that itseems to beof very ancient formation and has undergone an elevation of15 to 20 feet.

The

island is flat,

composed

almostentirely of

naked

coral rock, rough

and

jagged, completely

honeycombed

in everydirection with pitsand fissures. Scarcelyanysoil exists,except ing where a small quantity ^of rich mold, formed

by

decomposingcoral,

hasaccumulated in hollow^rs of therock.

The

.sea face is an overhang ing cliff of rock, but in a few places, especially on lie Picard

and

on the west coast, are sandy beaches

and

low sand-hills.

Upon

the south coast are

Dune

Jean Louis

and Dune du

Meche, sandhills,whichreach G5 feet above sea level

—

the highest points in Aldabra.

Nearly the whole surfaceis covered with a dense, almost impene- trablescrub of tangled bushes.

No

large trees

now

exist except the mangroves, which attain a height of (10 feet and a diameter ^of a foot ormore. Formerly

some

large trees existed, as

shown by

the decay- ing stumps and fallen logs, occasionally 2 feet in diameter, still to be found upon lie Picard. In

some

places the larger

mangroves

aredead over areas of several acres.

The

disappearance ofthelarger trees can onlybeattributed to a diminution inthe rainfall.

The

supply offresh wateris very scanty, only obtainable in hollows in the rock, except at one place nearthe southeast corner of

Grande

Terre. Here there is a sort of spring, filling a hollow in therock 6

by

2 feet, and 5feet deep. This seems to drain a considerable area, as the level of the water can not be appreciably lowered by balingout.

The

water is of poor quality.

The

rainfall is scanty

and

veryirregular.

Sometimes many months

elapse during which not a drop ofrain falls, and, on the other hand.^(J inches have been registered in a single night.

The

^islets in the lagoon are of very peculiar form, generally

more

or less

mushroom

shaped.

The

level of their flat

summits

is a few feet

levelof preaenl reef.

Fig. ^i. Sectiou ofanisle< in the lagoon.

above that of spring tides.

They

are*evidently the remains of the ai'.cient floor of the lagoon. All the other parts having been

cutaway

by the action of the water, the sides of all are undermined, and the smaller frequently present a very perfect mushroom-shape, as

shown

^1893^']

PROCEEDINGS OF THE NATIONAL MUSEUM.

761

in the

accompanying

figure. Sometimesthe top is30 feet in diameter, perched

upon

asupport of 5 or 6 feet in thickness.

As

before stated, the island ^is full ofpits. These are often 20to30 feet in diameter

and

as

many

feet deep,

and

are full of salt water at high tide.

Near

the western end ofthe floorof thelagoon is a large hole,through which thewater spouts

up

as thetide rises. This open- ing doubtless communicates with the sea outside through subterra- nean passages,

and

as the tide outside thelagoon is oneor

two

Lours inadvance of thatinside, this

phenomenon

iseasily accounted for.

The

currentssweepwith great rapiditythroughthe lagoon,especially near the channels, but in

some

of the calmer corners, particularly in the southwest, the bottom is covered with a layer of line white

mud,

similar to that described

by Darwin

at Keelingatoll.

The

average temperature on the island during October

was

^76° in

earlymorning

and84°

duringtheday. After the

monsoon

changed,early inDecember,it

became much damper and warmer — up

^{to90°intheshade}

at 2 p. m.

During

October

and November we had

norain,the vegeta- tion

became

quitedried up,

and

mosquitoes wereabsent. In

December

about15 inches ofrainfell;vegetationawakened, nearlyevery plantput forth fresh green leaves

and

flowered.

A ^more

complete transforma- tion could scarcebeimagined.

The

desert island

became

ablooming garden filled with theperfume of flowers.

The most

remarkableindigenous inhabitant of

Aldabra

isthe gigan- ticland tortoise,* similar to those oftheGalapagos group.

They

were formerlyvery abundant, butbeing easily caught

and

in great

demand

for their flesh, their

numbers

have been greatly diminished by the whalers

and

fishermen visiting the island.

They

are

now

protected (nominally)

by

the

government

ofSeychelles, towhich Aldabra belongs.

They

are^stillfound

upon Grande

Terre

and

lieNord, probably in con- siderable numbers, although I

met

with butfew, as

many

parts of

Aldabra

arewholly inaccessible,

owing

totheruggedsurface

and

dense jungle.

They were

completely exterminated

upon

lie Picard about twelve years since,but haverecentlybeen reintroduced

by

thepresent lessee of the island, Mr.

James

Spurs.

At

the present

day

they are

more

plentiful in the Seychelles than in their original habitat.

They

were brought

many

years since tothe formerislands,

where

they breed freelyinconfinement,

and

are

much

valuedforfood,being eaten atmar- riage feasts

and

onotherfestive occasions. Itistheonlyremainingspe- ciesofthe giganticland-tortoisesthat formerlyinhabitedBourbon,Mauri- tius,

and

Rodriguez(andprobablyalsoMadagascar)atthetimeoftheir discovery.

A

single individual, probablyoftheRodriguez species,still livesat Fort

George

Barracks,in Mauritius.

The

greatest

enemy

of the land tortoiseis the

common

^rat, which

swarms

upon Aldabra

and

eats the

young

as soon as they are hatched.

The

only other laud-reptiles

upon Aldabra

are a smalllizard {Able- Probably Testudoelephantina. F.A. L.

7G2 NATURAL HISTORY OF ALDABRA ISLAND— ABBOTT.

pharuspcecilopleurus) and

two

geckos(Hemidactylus mabouia

and

I'ltvl-

swna

abbotti*).

Turtles areplentiful.

Many

thousands annually ascend the sandy beaches to deposit their eggs. Tortoise-shell

was

formerlygatheredin large quantities,butthis fishery hasbeen overworked and large"carr6"

arc

now

scarce.

Mammals

^are represented by a large fruit bat (Pteropus aldabrensis, True),

and

two smaller bats,t Rats

(Mus

decumanus), probably from wrecked vessels,

swarm

everywhere,

and

are very destructive. Tats, probably from the

same

source, are

common

upon

Grande

Terre, where they havecompletely exterminated theflightlessrail.

Land-birds are represented

by

fourteen resident

ami

six accidental or visiting species; Mater-birds

by

twenty-four species. Doubtless

many more

occasionallyvisit the island from

Madagascar and

Africa.

The most

interesting species of birds is the curious flightless rail

(Rougetius aldabrantts,Eidgway),the sole survivor of the

numerous

flightless birds that inhabited the Mascarine Islands at thetime of theirdiscovery. I fear the presentspecies

must

follow theirexample, as their arch enemy, the cat,has already exterminated

them

from

Grande

^Terre. and must sooner or later reach the other smaller islands of the group,where the rails as yet

abound

in great numbers.

The

other land-birdsareapparentlysimilarto,or identical with,

Madagascar

species.

Boobies of several species,| frigates (Fregata aquila minor),

and

various speciesof terns§

and

sandpipers, abound.

A

flamingo (Phcenicopterus erythrceusf) isfound in considerable

num-

bers. Thisis particularlyinteresting as havingalso existed in

Maun

tins attimeof its discovery.

Fish are notveryplentiful inthe neighborhoodoftheislands.

Huge

cocoanut-crabs (lobsters) abound, as alsoland-crabs.

Insects are not

numerous

either in species or individuals. Six or seven butterflies, a few moths, a dragonfly, a fewbeetles,

some

^flies,

and

bees arefound.|| Mosquitoes

abound

New speciesdescribed by Mr. Stejneger.

tOncof these is Nyctinomus pumilvs. F. \Y. T.

!<>nh SulapiscatoriLinn.* isrepresented in the collection madeby Dr.Abbott.

—

R.R.

§Sternabemsteini,S.fuliginosa,S. melanauchen,Anon*stolidusandGygiaalba.

—

R. R.

IIMr. Linell furnishes the following list <>i' Aldabra insects received from Dr.

Abbott:

Butterflies:

1. Diadema misippiM,^1>. Bothsexes taken; .1,blackwithviolet-sholwhitespots;

9,brown withblackand white wing-tips, closely imitating Danaischry-

xii>))its. The distribution of this species is remarkable. It is rare in

AmericafromSouthFloridathroughtheWestIndiesto^tlieAma/on region

;

more common in Africa (except the Mediterranean region)andthrough SouthernAsia andthe Malay ArchipelagotoNew Holland,

2. Junonia delta, Cram.

Common

in South and Easl Africa.

vor..xvi,

1893 ]

PROCEEDINGS OF THE NATIONAL MUSEUM. 7G3

Theislandsare covered with dense scrub, mostly

composed

ofshrubs 4-to8 feet high.

No

large trees except

mangroves now

exist,

and

small plants areremarkablyscarce. Thereareno ferns or orchids, but considerable quantities of Orchilla

moss

are gathered. Formerly

"porche"

and

- boisrose"

grew

to considerablesize, judging from the

iirSr ^Umi,Samll

^{° §}

'

8

'

A ^

^{C°COaimt tre6S exist>mostly}

^upon

Aldabra is not permanently _inhabited, but there are usuallv a few fishermen from _Seychelles living there.

The

whole

Aldabra

grout) includingAldabra, Astove, Assumption, and Cosmoledo, _{belongs to} the British colony of Seychelles,

and

is leased

by

the

Government

tor theturtle

and

tortoise-shell _fishery.

ASSUMPTION

ISLAND.

Assumption

lies;*)miles southeast of

Aldabra and

isabout5miles long

by

1^ miles wide. Inits physical features itis

much

like

Aldabra

_but

its surfaceis smoother

and

it isnot so densely covered withscrub'

A

considerable part of the surfaceis covered with

"champignon

^"_{as the} ruggedfossil coral rock of

Aldabra

iscalled. Thereare

two

large sand dunes

upon

the easternshore, about70feethigh,whicharevisiblefrom a considerable distance.

No

fresh water exists, unless just after a ram,

when

a littlecollects in hollows inthe rock.

The

animal _life

and

vegetation is similar to that of Aldabra.

The

little flightless rail (Rougetiusabbotti,

Eidgway)*

abounds, _{as well as} most of the other land-birdsfound

upon

thelatter island.

Numbers

_{of goats run wild} having been introduced

many

years since from

Europa

Island fin

Mozambique

Chaunel). ^v

GLORIOSO

ISLAND.

Glorioso Island liesabout 90 miles west-northwest of the north point of

M^igasc^CCape^

_{Amber), and 120} southeast ofAldabra. It is 3. Lyorna UUeanus,Hub. South Europe through East Africa to theCapeof

(jroodHope. ^{• *}

Dragonily

:

1. Pantalaflavescens, Pah. A common East Africanspecies Myrmeleon. _{Myrmeleon,sp.}

Mantid:

1. rohjspilota variegata, Oliv. An East Africanspecies Beetles:

1. Oxythyrea_amabilis, Schonh.; var. Smallerthanthe continentalforms

-. Smallscaraband—undetermined.

3. Small longicorn— undetermined.

Wasps

:

1. Sphex,sp.

2. MonQdula,sp.

Otber diptera

:

Qdantomyia, sp.

wt^T*

^{8PeCie8 fr°}

^m

^{tba* °fAldabra' d}

— ^iW

»'

«•

^A«*> for'^'naiy,

764 NATURAL HISTORY OF ALDABRA ISLAND —

^ABBOTT.

situated

upon

the south end of the Glorioso bank.

The

bank ^is about 8 mileslong

by

2 in width, tie

du

Lise lies at the north end of the bank.

Glorioso Island containsabout700acres,being¹;\miles

longby

1mile wide. It is partly covered with sandhills50to60feethigh. Formerly

it

was

almost entirely covered with a growth of "porch6," "bois lose," "fouche" and other large trees, but atpresent

many

have been cut down.

The

soil is unusually fertile for an oceanic island, having been

manured

for ages

by

thousandsof sea-birds.

"Champignon,"

or

fossil

—

coral lock,such as composes Aldabra,⁽'osmoledo,etc.,exists in only a few ^spots, and the soil or sand ^is of fair depth. Large quanti- ties of maize are grown.

Water

from wells is of poor quality. There are live species of land-birds, three of which, a sun bird,* Zosterops,]

and abulbulj areprobably ^peculiar.

i oininon fowlsrun wild inthe jungle inconsiderablenumbers.

They

areveryshy and noteasyto shoot.

Among

^sea-birds thereis a booby, which seems to be peculiar to the island.§

They

breed in large

num-

bers

upon

the "fouche" trees, in

company

withfrigates

and common

boobies.

Upon

the neighboring small islet of Lise vast

numbers

of

"Wide- awake"

terns (Sternafuliginosa)breed, together with ''General" and

"Capucin"

boobies (Sula cyanopsand 8.piscator).

A

gecko (Hemidac- tylus mabouia) and two otherlizards, (Ablepharus gloriosus Stejneger, Zonosaurus madagascariensis) are plentiful.

Numbers

of wild cats range the jungle, so that birds are far less numerous, individually, than in Aldabra.

*Cinnyrissouimanga.

\Z. madagascariensis.

\ Txocincla madagascariensisf

vS

Two

species of boobieswerecollected byDr. Abbott, sula cyanopsandS.piscator

)

lmt bothof theseareof very widedistribution.

—

R, R.